Ayaan Hirsi Ali has called the prophet Muhammad a "lecherous tyrant", Islam a "backward religion", and the Koran "in part a licence for oppression". Theo van Gogh dubbed Muslims "goat-fuckers", a radical Islamic leader "Allah's pimp", and Islam a "retrograde and aggressive" faith.

Van Gogh, the 47-year-old great-grandson of Vincent's brother and a talented if wildly provocative film-maker, columnist and TV interviewer, died on a street in eastern Amsterdam on Tuesday morning, slain by a suspect whom police yesterday described as an Islamic fundamentalist with terrorist ties.

"I feel terribly guilty," a shocked Hirsi Ali told Dutch media yesterday, adding that she was "very much afraid" that Submission, an 11-minute film about Islamic violence against women that she wrote and the film-maker produced, was the direct cause of his death. Unlike van Gogh, Hirsi Ali lives under 24-hour police protection.

The elegant 34-year-old MP for the free-market VVD party, a Somalian refugee who 12 years ago fled an arranged marriage and now calls herself an "ex-Muslim", has every reason to be distressed: the manner of Van Gogh's death was brutal - and, it emerged yesterday, depressingly familiar.

The film-maker was shot several times as he rode on his bicycle down the Linnaeusstraat to his office, but still managed to stagger some distance - 30 or 40 metres, witnesses said - before being caught in a second hail of gunfire by his attacker, a 26-year-old man with joint Dutch and Moroccan nationality. On his knees, the eyewitnesses said, Van Gogh twice begged for mercy. But the suspect, described as having a beard and wearing a long jellaba, fired again and then drew two butcher's knives, slitting his victim's throat before driving the blades into his chest. Police found a letter on the body, but have yet to reveal its contents.

The Dutch justice minister, Piet Hein Donner, said yesterday that the suspect, captured after a shootout with police and currently in a prison hospital with gunshot wounds, "acted out of radical Islamic fundamentalist convictions" and had contacts with a fundamentalist group that was under surveillance by the Dutch secret service. Dutch media also reported that the suspect was a close friend of Samir Azzouz, an 18-year-old Muslim of Moroccan origin who is awaiting trial on charges of planning terrorist attacks on targets including a nuclear reactor and Amsterdam's Schiphol Airport.

The assassination has sparked a heartfelt national outcry in the traditionally tolerant Netherlands, sparking fears of a dangerous rise in racial tension in a country whose population of 16 million includes some one million Muslims, mainly of Turkish or North African origin. Fanning fears further, a recent government estimated that by 2010, several large Dutch cities like Rotterdam, Amsterdam, the Hague and Utrecht would have Muslim majorities.

Recent opinion polls show the Dutch to be increasingly hostile towards immigrants and fearful of Muslim extremism. Islam, immigration and integration have shot to the top of the political agenda since the rise of Pim Fortuyn, the populist anti-immigrant politician who was himself shot dead by an animal-rights activist in May 2002, and whose party finished second in general elections just days later. The centre-right Dutch government has only succeeded in fanning the flames by calling for greater integration of immigrants through language tests and citizenship classes, and recently fuelled even more controversy with plans to repatriate up to 26,000 failed asylum seekers.

In the midst of this tinderbox, insisting on their right to speak freely and with the support of many Dutch people, Hirsi Ali and Van Gogh scattered their sparks - a blistering critique of Islam - with magnificent disregard for the feelings they might be offending.

The slender, couture-clad Hirsi Ali has had several fatwas issued against her and spends her life in the company of a brace of six-foot bodyguards; Van Gogh also received death threats but refused protection, saying the bullets would surely never come for him. "No one can seriously want to shoot the village idiot," he said recently.

Their film was broadcast on Dutch national television in August. It depicts, among other scenes, a beautiful young Muslim girl addressing Allah in a mosque. She wears a veil that covers her face, but her naked body is clearly visible through a transparent gown.

"All praise to Allah, the Lord of the Worlds," says the text that scrolls across the actress's throat and down her breasts: the fatiha, or opening of the Koran. Other scenes portray a Muslim woman who is forced into an arranged marriage, abused by her husband, raped by her uncle and then brutally punished for adultery.

In a third, a woman's bruised and beaten shoulders are covered with lines from verse 34, chapter 4 of the Koran. "Men are the maintainers of women because Allah has made them excel ... " it reads. "The good women are therefore obedient. Those on whose part you fear desertion, admonish them, and leave them alone in the sleeping-places, and beat them."

The film was a potent, if undeniably provocative, interpretation of Hirsi Ali's thesis. Brought up as a Muslim in Somalia, she suffered female circumcision at the age of five and, sent to Germany to meet her intended Somali partner in an arranged marriage, fled across the border to the Netherlands in 1992. Penniless, speaking no Dutch, she worked as a cleaner, in a biscuit factory and as a translator before studying political science at Leiden University.

In 2001, after graduating, she wrote a report on "honour killings" of Muslim women that also served as a savage indictment of Holland's 30-year experiment with multiculturalism, describing it as a "disastrous error" born of "misplaced guilt". The report embarrassed the Dutch Labour party, which comissioned it, but the VVD - which has a tough "boat is full" stance on immigration - welcomed her with open arms, first as a researcher, then as a candidate. She has sat in The Hague as an MP since January 2003.

On TV talk shows and in newspaper columns, Hirsi Ali has denounced the "cruelty and abuse" meted out to many Muslim women living in western societies.

Damning Islam as a "backward, 12th- century religion", a "medieval, misogynist cult incapable of self-criticism and blind to modern science", Hirsi Ali says orthodox Muslim men routinely indulge in domestic violence against women, as well as incest and child abuse. To make matters worse, she argues, their behaviour is invariably hushed up.

"The Netherlands is a country that worships consensus and peace, but here you have newcomers who are not integrated into this system," she said last year. "They exploit an open, liberal society to reach illiberal ends. Everyone knows the position of women in Islamic countries is horrendous, but the Dutch like to think it doesn't happen here. They don't want to believe Muslim women in the Netherlands are beaten and locked up in their homes, or that girls are murdered for holding hands with a non-Muslim boy."

The solution, Hirsi Ali argues, is for fundamentalist Islamic books to be banned, Mullahs to be banished and for western societies "not to bend over backwards to accommodate a culture that advocates the degradation of women ... but to ensure that the Muslim men who perpetrate such barbarity are brought to justice".

The "lapsed Muslim" last year found an effective and articulate artistic partner in Van Gogh who, as well as having made a dozen feature films in his 25-year career, was also a much-loved, deliberately provocative and often obscene columnist and pamphleteer who published numerous indictments of an over-radical Islam in an over-tolerant Netherlands. Fired over the years by almost every Dutch newspaper and magazine for offending its readers, he wrote most recently for the daily freesheet Metro and ran his own highly popular website, De Gezonde Roker (The Healthy Smoker).

But in the no-longer-tolerant-Netherlands, he paid the price. Fraught Dutch commentators had no hesitation yesterday in saying that Holland had become a "front-line state" in a brutal collision between two cultures. "In France or Belgium, you don't have this same kind of very Dutch cabaret-like figure who rages about goat-fuckers," one commentator, Rene Cuperus, told De Volkskrant.

"They must know that they've landed up in the most liberal country in the world, the land of abortions and gays and all that - but Muslims don't see it. There's just no way to bridge that gulf in a politically correct way." Sociologist, Herman Vuisje, described Van Gogh's murder as "not a turning point, but the after-effect of a historical failure". And an academic, Norbert Both, posed the question that, one imagines, is now troubling Ayaan Hirsi Ali - as well as a great many less outspoken Dutch people.

"The great dilemma, in confronting intolerance, is that you cannot reply with tolerance," he said. "But if you answer intolerance with intolerance, you lose your own identity. Can we, despite the emotion, remain ourselves? That's the question."